A comprehensive study of surface and space charge effects at an operating water splitting iron oxide photoanode

Abstract

This work presents an investigation of the charge distribution across an operating, water splitting α-Fe2O3 photoanode from bottom up. The study covers the entire experimental framework from the preparation of a representative sample system via the construction of an appropriate sampling environment up to the direct detection and analysis of surface- and space charge at the electrode surface using second harmonic generation (SHG) spectroscopy in combination with photo-electrochemical standards. All results are constantly validated by a range of control measurements. It is shown how the progress of galvanic FeOxHy film deposition can be modified and monitored by the deposition potential and current, respectively. In the alkaline water photo-electrolysis current-voltage characteristics, a small capacitive signal is detected from the α-Fe2O3 photoanodes and discussed in terms of electronic surface state charging in good agreement with literature values. On the base of a kinetic analysis of cathodic photocurrent transients, a third order water oxidation rate law is inferred. By means of complementary density functional theory calculations, a photo-electrochemical oxygen evolution reaction mechanism is suggested and discussed in terms of surface charge accumulation processes. An SHG signal is measured and ascribed to the α-Fe2O3 photoanode material that can be modified by the external bias potential along both the intensity and spectral dimensions. The SHG response is shown to correlate with the band bending at the electrode surface according to the electric field induced second harmonic (EFISH) theory in good agreement with photo-electrochemical analyses of the sample system. With this, it is possible to directly map and analyse surface the band-bending as function of the applied potential. Considering the donor density in the semiconductor electrode material, a complete majority carrier depletion of the bulk semiconductor system at higher potentials is inferred in good accordance with findings on the morphology from electron microscopy and X-Ray diffraction. The concomitant transition from a quadratic to linear potential dependence of the SHG response is modelled in terms of a transition from purely depletive to dielectric electrode polarization in series with a semi-conductive back contact. Spectral changes of the potential dependent SHG response are interpreted analogously and considered an indication of an additional band-component to the semiconductor electronic structure.